Sensor elements for pressure sensors based on ceramics and constructed as dilatation or strain sensors or capacitive sensors can conventionally comprise various ceramic materials. Often, then, ceramics is used based on aluminum oxide but also glass ceramics is used. There are some problems associated with the use of such sensors and some of them will be discussed hereinafter with reference to FIG. 1 and FIGS. 2a-2d.
A capacitive sensor element 1 of conventional type based on ceramics, for example glass ceramics, is illustrated in the sectional view of FIG. 1. The sensor element comprises a thick house part 1, in the shape of a plate 3 comprising an annular projection or platform 5 located at the circumference thereof on one side. At the annular projection a thin plate 7, a pressure diaphragm, is attached, which thus has a considerably lesser thickness than the house part 1. The house part 3 and the diaphragm 7 carry on their interior, opposite surfaces at the central portions thereof electrically conducting areas 9 and 11 respectively in the shape of thin layers, which normally have the same configuration and are located opposite each other and at a small distance of each other. When the distance between the opposite surfaces on the two conductive areas 9, 11 in the capacitor formed thereof is varied, the capacitor will have a varying capacitance, which is readily detected by means of suitable electronic circuits. For an optimally designed sensor element then the capacitance or some function derived therefrom in a simple way, for instance the inverted value of the capacitance, should be a linear function of for example the distance of the plates in the capacitor formed. The sensor element can then be used for pressure measurement and then, in the corresponding way, the electronically detected capacitance or some other quantity derived in a simple way should be proportional to the pressure acting on the diaphragm 7. However, deviations from the linear behavior are always present and will be described hereinafter.
The presupposition that a linear function exists or should exist, is based on the theory of the capacitance between two flat electrically conducting plates which are parallel to each other. However, for the pressure sensor according to FIG. 1, the movable diaphragm 7 and hereby the electrode plate or the electrode area 11, which is located thereon, will have some curved profile, when the diaphragm is exposed to an exterior pressure deviating from the pressure on the chamber formed between the house 3 and the diaphragm 7. The electrodes 9, 11 in the plate capacitor formed are thus then not flat and not parallel to each other. This deflection effect can be calculated numerically and has generally a small importance in a complete or finished pressure sensor having the sensor element attached in a house. If required, however, this effect can be compensated in an electronic way.
Another basic condition for the presupposition of a linear dependence of an output signal and the pressure acting on the diaphragm is that the magnitude of the deviation or deflection of the diaphragm is proportional to the applied pressure force. It is valid for small deviations from the equilibrium position of the diaphragm, where the equilibrium or rest position can be the state of the diaphragm for equally large pressures on the two sides thereof. However, for larger deflections from the rest position the deflection will not follow the pressure force proportionally but is less than what would be obtained in an ideal proportional case. The deflection of the diaphragm can be calculated in different ways, such as for example be calculated approximatively by means of a theory such as "Large Deflection Theory, LDT". This non-linear effect can be eliminated by a correct dimensioning of the diaphragm, for limited pressure ranges, and a good resolution of the electronic circuit which detects the electrical quantity which is a measure of the pressure. Practically, this effect occurs especially in very thin diaphragms, since thick diaphragms where the thickness of the diaphragm is considered in relation to the height of the deflection, break before they reach the non-linear deflection range.
The dominating cause of deviations from a non-linear behaviour of capacitive sensor elements according to what has been discussed above, is however stray capacitance of various kinds. There are both edge effects at the edges of the capacitor plates and capacitances in relation to other electrically conductive surfaces and areas adjacent the capacitor electrodes. These effects thus produce deviations from the linear behaviour of an output signal from the sensor element which are definitely outside what can generally be accepted by a user of precision pressure sensors. The influence of the various stray capacitances is complicated and can be described as combinations (for example sums) of functions of different kinds, a constant function, a linear function, non-linear functions of various kinds such as exponential functions, etc.
The influence of stray capacitances can be divided both in the fact that they affect the maximum value of the deviation from a linear behavior and a deviation curve having different profiles over the measuring range of the sensor. Different forms of deviations from the linear behavior are illustrated in the diagrams of FIGS. 2a, 2b, 2c and 2d. The deviation is shown as a function of the applied pressure between the value 0 (corresponds to the rest position) and the value indicated by FS (="Full Scale"), which designates the upper limit of the measuring range. For a digitally working processor it is naturally possible to compensate these various deviations but for simpler electronic components of a more robust type, difficulties can be obtained in the compensation procedures.
The desired compensation signal, which is to be superposed on the deviation from a linear behavior of the output signal can be described as a polynomial of the deviation from the normal position of the input signal, for instance of the pressure. For simpler electronic circuits, terms in the compensation signal can be achieved up to and including the quadratic term. It is more difficult to use functions having higher degrees when using electronic components of standard type. In such a linear compensation having at most quadratic terms it facilitates significantly if the deviation curve is symmetrical, see FIG. 2a. The maximum magnitude of the deviation can be adjusted by means of adjustment of constants in the compensation function. Generally, however, the case is that a large value of the maximum deviation also results in an increased asymmetry of the curve, which for a good compensation, also if the deviation from the linear behaviour is symmetrical in accordance with FIG. 2a, a deviation having an S-shape is obtained, compare FIG. 2d, in the compensated output signal. This effect is however most often of a more theoretical nature and will generally not be observable to a user of the pressure sensor.
The magnitude and the shape of the curve of the deviation of the output signal from a linear dependence of the input signal can be influenced by designing the sensor element in different ways. It is conventional art to coat the sensor house part and the diaphragm part with an electrically conductive layer located on the outsides thereof, a shielding layer, which is connected to electrical ground. The layer of material can be of gold, platinum, silver, an alloy of silver and platinum, titanium nitride, tin indium oxide, etc. In the pressure sensor disclosed in the patent U.S. Pat. No. 4,935,841 the exterior, usually flat surface of the plate-shaped house part has been provided with a centrally located recess having a bottom surface which is located at a small distance from that electrode in the measurement capacitor, which is coated on the interior side of the house part. The exterior, grounded conductive layer is also coated in this recess and makes the electrical conditions around the capacitor electrode of the house part more uniform. Hereby deviation of the output signal from a linear behaviour will be effected both as to magnitude and shape.
When such a centrally located recess in the house part is provided, however, the sensor element obtains a reduced strength and the region at the bottom of the recess can even form a second flexible diaphragm, which will also in turn be influenced by the exterior pressure changes, for instance for a mounting of the sensor element where the load of the pressure which is to be measured is on only one side, that is, so that the pressure only acts on the measurement diaphragm, whereby this secondary diaphragm is influenced by the pressure changes in the surroundings, that is from the atmosphere. The movement of this secondary diaphragm will then cause incorrectnesses in the function of the sensor element. Incorrect functions can naturally also occur by the fact that ruptures appear at the connection or transfer region between the differently thick portions of the house part.
This sensitivity to changes of the exterior atmospherical pressure is for normal conditions small and is heavily extended in time. A user will not normally observe it. For extremely low exterior pressures, which can occur associated with very bad weather, a direct influence on the sensor element can be obtained. The mechanically weakening effect of the recess can largely be eliminated by filling the recess with a plug in the shape of a small cylindrical plate, which is attached in the recess by means of a suitable joining material, for glass ceramics a paste containing finely divided glass. Hereby, the house part of the sensor element will be stabilized mechanically. Such a sensor element will however have a complicated production procedure and will be subject to rupture at the recess.
A capacitive pressure sensor is described in the German patent application DE-A1 41 36 995 (Offenlegungsschrift). In the embodiment shown in FIG. 5, the interior surface of the diaphragm 4, the movements of which are detected, is coated with an electrically conductive shielding layer 9 by means of thin film methods. On the interior surface of this layer then a thin dielectrical layer 10 is deposited, which operates as a carrier or support of the diaphragm electrodes 7, 7'. The movable part will hereby be constituted by a rather complicated layered structure which will then present different characteristics as to elasticity and thus as to the movements, when the temperature varies, what results in that the detected quantity will have a dependence of temperature for which a prediction is difficult to make. It has also a negative influence on the zero stability of the measurement cell over long times.